This invention relates to thermally conductive pads and, more particularly, to a method for selecting the formulation of a silicone pad providing desirable elastomeric and thermally conductive characteristics therein.
Circuit designs for electronic devices have become more sophisticated resulting in various components closely packed on circuit boards. As a result, the problem of heat dissipation generated by component operation arises which raises the probability of component malfunction, if not failure.
Heat dissipation may be achieved by the use of a heat sink, whether in the form of a separate plate or device chassis, which is positioned adjacent the component for a heat transfer thereto. The interface between the respective faces may be irregular resulting in air spaces therebetween. As air is a poor heat conductor the spaces denigrate the heat transfer between the component and the heat sink. Thus, the operating temperature of the component may rise above its desired value.
This heat transfer may be improved by inserting a thermally conductive material between the component and heat sink so as to eliminate the air spaces therebetween. Such materials have been in the forms of waxes, greases and oils, inclusive of silicone materials loaded with heat conducting particles to increase conductivity. The silicone matrix needs to be very soft as the particle loading decreases its conformability, which in turn decreases the conformance of the material about the component face and the circuit board such that air gaps may result.
Such a soft material needs a supporting substrate, web or other carrier so as to maintain cohesion. However, carriers such as Fiberglas® or substrates prevent the material from being sufficiently elastic and so have limited elongation and flexion characteristics. Foam carriers that are impregnated with silicone and heat conducting particles have limited compression once the foam cells are collapsed. Such carriers, whether Fiberglas® cloth, foam substrate, etc., are unable to intimately conform to various contours of electronic components and so leave air gaps, which cause an increase in thermal impedance.
To eliminate the carrier and yet maintain conformability, oils and/or plasticizers are added. However, the resulting material may present poor thermomechanical properties. Moreover, when the component is compressed against the heat sink with the thermal pad therebetween, exudation of silicone oils into a monomolecular layer may result which contaminates the surrounding circuit board and components. This problem of silicone exudation has led some electronics manufacturers to forbid the use of silicone in thermal pads despite the desire to use such a silicone based material due to its conformance advantages. Accordingly, it is desirable to provide a silicone based thermal pad loaded with heat conducting particles which has the desirable cohesion, conformance, thermal impedance and elastomeric properties; does not require a substrate or carrier and does not exude undesirable amounts of oil when compressed in its functional environment.